The invention relates to a process for distilling an aqueous polymethylol mixture which is obtained in the preparation of polymethylols from alkanals and formaldehyde. The present invention further relates to a composition comprising polymethylol and 1 to 000 ppm by weight of polymethylol formate, and to the use thereof.
Polymethylols, for example neopentyl glycol (“NPG”) and trimethylolpropane (“TMP”), are used in the plastics sector for production of paint systems, coatings, polyurethanes and polyesters.
On the industrial scale, polymethylols are usually prepared by the Cannizzaro process. In order to prepare trimethylolpropane by this process, n-butyraldehyde is reacted with an excess of formaldehyde in the presence of an inorganic base. This likewise forms one equivalent of an inorganic formate as a coproduct. The separation of the salt of trimethylolpropane is complicated and requires additional work. Moreover, the inorganic salt—if it can be utilized in a profitable manner—must be worked up and purified. The occurrence of the coproduct otherwise constitutes a loss of the stoichiometrically used amounts of sodium hydroxide solution and formaldehyde. In addition, the yields in this inorganic Cannizzaro reaction are unsatisfactory in relation to n-butyraldehyde, since high-boiling constituents are formed in the course of the reaction, which cannot be utilized further.
Similar problems to those outlined for trimethylolpropane exist in the preparation of other polymethylols such as trimethylolethane (from n-propanal and formaldehyde) or trimethylolbutane (from n-pentanal and formaldehyde) or neopentyl glycol (from isobutyraldehyde and formaldehyde).
To avoid these disadvantages, WO 98/28253 disclosed a multistage process for preparing polymethylols, in which aldehydes having 2 to 24 carbon atoms are first condensed in a first stage (aldol reaction) with formaldehyde using tertiary amines as a catalyst to give the corresponding methylolalkanals, and then hydrogenated in a further stage (hydrogenation) to give the corresponding polymethylols. This multistage process is typically referred to as the hydrogenation process. This process is low in coproducts.
After the first stage of the hydrogenation process, unconverted aldehydes and a portion of the amine base are generally removed by distillation from the methylolalkanals formed and recycled.
In the distillation bottoms there remain—as well as the methylolalkanals formed—water, the adducts of formic acid and the tertiary amines used (amine formate) and formic acid itself.
In general, the polymethylolalkanal is obtained by these processes as a 20 to 70% by weight aqueous solution.
The aqueous polymethylolalkanal-containing solution is hydrogenated in a second stage in order to convert the polymethylolalkanals to the corresponding polymethylols, such as TMP or NPG.
In the course of hydrogenation, side reactions are generally observed, such as a redissociation of the methylolalkanal to free alkanal and formaldehyde and additionally ether, ester and acetal formation.
These side reactions lead to a low hydrogenation selectivity and to low yields of polymethylols.
These by-products can also impair the quality of the polymethylols formed and the use thereof in particular applications. For instance, formates present in the end product can decompose to form formic acid. Formic acid can, for example, catalyze the hydrolysis of urethane or ester bonds, which leads to accelerated aging of coatings and polymers.
In the context of the present invention, it has now been found that especially monoesters of formic acid and of the polymethylols formed (polymethylol formate) contribute to the formate content in the end product.
It was therefore an object of the present invention to provide a process for purifying an aqueous polymethylol mixture in order to obtain, polymethylols with a low content of polymethylol formates.
The object of the present invention is achieved by a process for distilling an aqueous polymethylol mixture which comprises a polymethylol of the formula (I)
in which each R is independently a further methylol group or an alkyl group having 1 to 22 carbon atoms or an aryl or aralkyl group having 6 to 22 carbon atoms, a tertiary amine, water and the adduct of tertiary amine and formic acid (amine formate), which comprises performing the distillation of the hydrogenation discharge in a distillation column which is connected at the bottom to an evaporator, the bottom temperature being above the evaporation temperature of the monoester of formic acid and polymethylol (polymethylol formate) which forms during the distillation.